The invention relates to methods and apparatus for homogenizing viscous material. More particularly, the invention relates to apparatus for mixing, stirring and blending molten glass as it moves through a flow path such as a forehearth channel or the like.
Molten glass is mixed or blended for a variety of reasons. For example, it is known that molten glass proceeding from a furnace or forehearth usually contains visible defects in the form of inhomogenities or discontinuities. These defects, known by a variety of names (stones, cords, seeds, striae, mares, and tails), result from impurities in the glass, incomplete fining or incomplete blending of color additives. If not sufficiently removed or dissipated, defects are carried over into the formed glass article, rendering the same unmarketable.
Glass is also stirred to remove thermal irregularities. When the temperature of the glass is uniform, it has predictable forming characteristics and better quality ware is produced.
Previous work with stirrers has resulted in satisfactory blending of glass. However, many stirrers may be required to produce good results. Further, there is usually some history associated with molten glass passing through stirrers. In other words, an element of glass entering the stirrer at a particular place will follow a statistically predictable path. Thus, unless many stirrers are used, the history of the element may not be completely wiped out.
History may be removed by slowing the flow rate of the glass, thereby allowing the glass to be subjected to the blending action of the stirrers for a longer period of time. When the flow rate is reduced, productivity decreases and energy consumption increases. Even with reduced flow rates and more stirrers, mixing is sometimes insufficient because some glass may find a flow path which avoids the stirrers. Thus, mixing does not occur and defects persist. The stirrer speed may be increased to improve mixing at high flow rates. However, there is a risk of seed and blister formation on the glass surface.
In the present invention, a pair of stirrers is provided for mixing glass in a channel or forehearth. All the glass passing through the stirrers receives sufficient mixing to produce good quality product. Mixing occurs in such a manner that the history of any individual element of glass is effectively wiped out. This means that a defect upstream of the stirrers will not reappear downstream thereof at a predictable location. If the position of the upstream defect is changed, there will be no observable change downstream of the stirrers. Thorough mixing occurs because the glass entering the stirring system sees a front or barrier. Care has been taken to effectively eliminate short circuit paths through and around the stirrers. This allows the glass to receive the full effects of the stirrers. Also, corresponding elements of glass, relative to a line of symmetry, receive the same treatment. Symmetry eliminates imbalances which cause short circuits.